In a combination reaction the atoms and/or molecules do what
the name implies: they combine to form the product. Combination
reactions are always exothermic. For example barium metal and
fluorine gas will combine in a highly exothermic reaction to
form the salt barium fluoride:

Another example is given by magnesium oxide combining with
carbon dioxide:

Often, reaction can fall into more than one category. For
example the combustion of magnesium metal is also a combination
reaction: